Spain had cruised to an easy 4-0 victory in the first leg on Friday and Tuesday's match looked destined to end goalless before Claudia Pina worked the ball into the box and then sent a cross to Putellas, who finished with a blistering shot into the net in the 75th minute.
Putellas and Pina had netted two goals apiece in the first leg.
In the two-legged final on November 28 and December 2, Spain will face either France or Germany, who play later on Tuesday, with the Germans having won the first leg 1-0.